1. Field of the Invention
The present invention relates to a sensorless control system and method of the permanent magnet synchronous motor and, in particular, to a sensorless control system which enables precise control and precise detection of rotor position and speed in a low speed operation of the motor.
2. Description of the Prior Art
The article "A Sensorless Detecting Strategy of Rotor Position and Speed on Permanent Magnet Synchronous Motor" by H. Watanabe et al. appearing in the Electric Society Transactions D, Vol. 110, No. 11, 1990, at pages 1193-1200, discloses an arrangement in which torque control of a permanent magnet synchronous motor (PM motor) driven with AC voltages into which an inverter has converted a DC voltage is achieved without using a sensor for detecting the rotor position angle. Specifically, the position angle and the angular velocity of rotor is obtained through a calculation based on the voltage equation of permanent magnet synchronous motor. A voltage command vector is calculated from the position angle, the angular velocity and a torque command given from the outside, and the inverter applies the AC voltages according to the vector command vector.
A phase winding voltage is necessary to calculate the rotor position angle and angular velocity on the basis of the above mentioned voltage equation, whereas there is a simplified system in which a voltage detecting device is omitted substituting the voltage command vector for it.
In this kind of motor control system, when a pair of series connected switching elements in the inverter is switched, in order to prevent the switching elements from shorting, the switching times of the switching elements are a bit staggered so as to provide a time period when both the switching elements are off (This time period is called a "dead time."). A phase current of the phase in a dead time is detected. The phase voltage of the phase in the dead time is estimated from the detected phase current and used for the calculation of voltage command vector.
However, the smaller the phase current becomes, the larger the estimation error of the phase voltage becomes due to the influence of ripple current etc. Especially if the magnitude of the voltage command vector is small, then the error between the voltage command vector and the actually applied voltage becomes large relatively to the voltage command vector, making it difficult or even impossible to calculate the precise position angle of the rotor.
Japanese Patent Unexamined Publication No. He12-179,277 (1990) discloses a compensation technique of the error voltage caused by the dead time of the inverter. In this technique, an offset current value is added to discretely obtained current values so as to make the current direction the same as the actual current direction. Determining the direction of the current based on the offset current value, a dead time compensation is achieved based on the determined current direction.
However, the above mentioned dead time voltage error compensation technique does not necessary perform precise dead time compensation, makes the system constitution complicated, and requires a expensive device such as an insulation amplifier for voltage measurement.
It is therefore an object of the invention to provide a sensorless control system and method of PM meter which is capable of precisely controlling the torque even if the voltage command vector is small or the motor is operated at a low speed.
It is another object of the invention to provide a sensorless control system and method of PM motor which is capable of precisely calculating the rotor position angle and angular velocity by precisely compensating a voltage error due to a dead time with a simplified arrangement without the need of an expensive device such as an insulation amplifier.